Effect of Zn addition on the microstructure and mechanical properties of Mg-0.5Ca-0.5RE magnesium alloy

The microstructure and mechanical properties of Mg-Zn-0.5Ca-0.5RE magnesium alloy, with zinc contents of 0, 2, 4, and 6 wt%, were studied. A significant effect of Ca and rare earth (RE) elements (via mischmetal addition) on the grain refinement of cast ingots was observed with the consequent enhancement of ultimate tensile strength (UTS). Moreover, while the grain size was refined by the addition of Zn, no significant alpha-Mg grain refinement occurred beyond 2 wt% Zn addition. The Mg12RE intermetallic compound was observed in all alloys while the Mg7Zn3 phase formed at the grain boundaries of Zn-containing alloys. Based on the competition of grain refinement as a favorable factor and formation of unfavorable grain boundary phases, the Mg-4Zn-0.5RE-0.5Ca (ZEX400) alloy showed the highest UTS among the cast alloys. Intense grain refinement introduced by the extrusion process and the fracturing and dispersion of the grain-boundary phases resulted in the significant enhancement of strength and ductility and the appearance of ductile fracture surface for the extruded Mg-4Zn-0.5RE-0.5Ca alloy. It was found that high Zn content (similar to 6 wt%) limits the usability and hot working temperature range of the alloys due to the problem of hot shortness and intergranular brittle fracture. Therefore, the addition of 4 wt% Zn was recommended for industrial practice. This work revealed the significant effects of alloying (with Ca, RE, and Zn) and hot extrusion process for microstructural refinement and enhancement of mechanical properties of Mg alloys. (C) 2019 Elsevier B.V. All rights reserved.